1. Field of the Invention
The invention relates generally to methods for measuring work function and, more particularly, to methods for measuring work function of field emission material.
2. Discussion of Related Art
Work function is the minimum energy needed to remove an electron from a metal to a critical point immediately outside the metallic surface. The work function is an important property of the metal and is usually measured in electron volts. Generally, the measurement for work function of the metal is important for the research of electronic emission properties of the metal. Commonly, the electronic emission of the metal includes thermionic emission and field emission. In the thermionic emission, the electron gains its energy from the heat when the metal is heated to a relatively high temperature. In the field emission, the electron gains its energy from the electric field and is removed from the metallic surface because of an electronic tunneling effect.
A conventional method for measuring work function is adopted in the thermionic emission for measuring the work function of the metal. Specifically, a metal emitter to be measured is provided as a cathode electrode with a spaced anode electrode in a vacuum diode. The metal emitter is heated and an outer electric field is applied between the anode electrode and the cathode electrode. When the metal emitter is heated to a relatively high temperature (about 1000° C.), a plurality of electrons may overcome the work function barrier of the metal emitter and escape outward from the metallic surface. Further, the escaped electrons may be moved from the cathode electrode to the anode electrode. The continual thermionic emission in the electric field will make a continual electric current produced between the two electrodes. Furthermore, the Richardson-Dushman equation relates the current density of a thermionic emission to the work function (W) and temperature (T) of the emitting material is expressed as follows:js=A×T2×exp(−W/kT);
wherein js is the current density of the emission (mA/mm2), A is Richardson's constant (approx. 1202 mA/mm2K2), T is the temperature of the emitter (K); W is the work function of the emitter (J), and k is the Boltzmann constant (1.38066E-23 J/K). According to the Richardson-Dushman equation, the work function of the metal emitter can be calculated by the values of the T and js, which can be measured directly. Apparently, a good thermionic emitter has to have a combination of properties including a low work function and a high operating temperature. However, metals with higher melting points typically have a higher work function, which is bad (i.e., not conducive) to the thermionic emission. As such, the thermionic emission method can usually be applied in measuring a work function of just a few kinds of metal, such as tungsten and so on. Still furthermore, the work function cannot be accurately measured when different kinds of metal emitters are measured in the thermionic emission method.
What is needed, therefore, is a method for measuring work function that has a relatively low operating temperature and that allows the work function of a plurality of metal materials to be measured accurately.